Hello Cai,
While it is certainly possible that the patches for 596223, there are a lot of patches that went in between -37 and -42. Could you try the intervening kernels (-38, -39, -40, and -41) to narrow down where the problem started happening?
Also, while this is certainly a problem on the T400, this problem doesn't seem to happen across the board in my testing. Could you repeat your tests on another Intel box to see if you see the problem elsewhere?
Unfortunately I'm going to be off until Wednesday of next week, so in the interim I will try to get somebody else to take a look at it. If we haven't figured it out by the time I get back on Wednesday, I will pick it back up.
Thanks,
Chris Lalancette

> While it is certainly possible that the patches for 596223, there are a
> lot of patches that went in between -37 and -42. Could you try the intervening
> kernels (-38, -39, -40, and -41) to narrow down where the problem started
> happening?
-38 had the problem.
> Also, while this is certainly a problem on the T400, this problem doesn't
> seem to happen across the board in my testing. Could you repeat your tests on
> another Intel box to see if you see the problem elsewhere?
Correction - saw it on a X200 laptop not T400. I'll try the T400 next Monday.

Sorry Chris, it is not your patches' fault. I have narrowed it down by pulled out the following 2 patches from -38.el6 kernel, everything is working fine again.
- [virt] account only for IRQ injected into BSP (Gleb Natapov) [601564]
- [virt] KVM: read apic->irr with ioapic lock held (Marcelo Tosatti) [579970]
I am compiling another kernel to find out which one is at fault.